It is needless to say that air plays an important role in the living environment of humans. With air are associated various parameters, such as the temperature and humidity thereof and the substances contained therein, and these parameters determine how comfortable and healthful it is to humans. Substances that may be contained in air are wide-ranging, examples including, in addition to dust, which is present everywhere, industrial pollutants such as fine mineral, metal, and other particles and exhaust gases, pollens and spores, microorganisms, odor-causing molecules, and carbon dioxide contained in exhaled breath.
In regions blessed with a favorable natural environment, it is largely possible to obtain comfortable air by natural ventilation alone. However, in regions with a poor outdoor environment, it is necessary to condition air artificially by some means to obtain comfortable air. Moreover, modern houses are increasingly built air-tight, which trend has been contributing to a greater demand for the conditioning of indoor air.
In conventional conditioning of air, the removal of airborne unpleasant or hazardous substances is achieved typically by filtering, absorbing, or decomposing them by means of a filter. However, filters are subject to poorer performance after an extended period of use, and thus inevitably require some form of maintenance such as replacement. In addition, it is difficult to ensure sufficiently high filtering performance to trap airborne bacteria effectively.
One important factor that determines the quality of air is the presence of ions in it. In particular, negative ions have been recognized to have a relaxing effect on humans. However, negative ions diminish as they bond to particular substances. For example, in the presence of cigarette smoke, negative ions may diminish down to about ½ to ⅕ of their normal concentration. To compensate for this loss, as a means for artificially augmenting negative ions in air, negative ion generators have been developed and put on the market.
Japanese Patent Application Published No. H7-23777 discloses an air conditioner in which a high alternating-current voltage is applied to a discharge needle unit provided in an air flow passage to generate negative ions so as to compensate for the negative ions that diminish as fine particles of pollutants increase in a room. Here, the use of a high alternating-current voltage prevents the product itself from being charged in an unbalanced fashion, i.e. either positively or negatively, and thus prevents the dust in the air inside the room from settling on the product. Moreover, the negative ions generated exert an relaxing effect.
In the air conditioner disclosed in the aforementioned application, however, the discharge needle unit is disposed near an air outlet located on the downstream side of a heat exchanger so that the ions generated are blown out into the room by a flow of air produced by a blower. Thus, in cooling operation, cool air directly hits the discharge needle unit. This may cause the discharge needle or another electrode to become frosted, leading to a short circuit. Moreover, the flow of air is disturbed as it hits the discharge needle unit. This may cause uneven blowing of air and a loss in the volume of air blown out, both undesirable effects in an air conditioner.
On the other hand, when an ion generating device generates ions, it simultaneously generates ozone as a byproduct. A high concentration of ozone is hazardous to the human body, such as by affecting the respiratory organs. This makes it necessary to devise some countermeasure against ozone.